[Noisebridge-discuss] Disaster Recovery SDR-based mesh

Marc Juul juul at labitat.dk
Fri Sep 13 22:00:06 UTC 2013

On Thu, Sep 12, 2013 at 11:47 PM, Matthieu Tourne <matthieu.tourne at gmail.com
> wrote:

> Hi!
> On Thu, Sep 12, 2013 at 7:33 PM, Marc Juul <juul at labitat.dk> wrote:
>> Hi!
>> Some of us at sudo room / east bay mesh are working on a low bandwidth
>> disaster recovery mesh project based on software defined radios.
>> We're prototyping with a combination of USRP1 and RTL-SDR devices and GNU
>> Radio.
>> We're having some issues getting the receivers to demodulate our data
>> streams (using DQPSK and QAM modulation).
> I never thought about doing much more than tuning to FM radios with my
> rtl-sdr, but that's a pretty cool project!
Thanks! You should come hack with us!

> I think that depends what's your goal, if you want to decode qpsk frame
> from classic radios [1] I think rtl-sdr can do it.
> If you want to Tx/Rx data accurately, I think you need a digital
> modulation chip [2]
> (my understanding is that it works best at the hardware level if you want
> a decent bandwidth)

The RTL-SDR is definitely not a high quality SDR, but we have very low
bandwidth requirements. For the transmitters we're only using USRPs while
prototyping. We're also working on a cheap transmit-only SDR circuit, but
that is definitely a longer term project and for now we just want a full
functioning prototype.

> Something like this [3] could help, if you know someone who can program
> fpga cards ..

That's lower cost than the USRP, but still more expensive than we'd like.

> But it's a pretty cool project, maybe you can use an OFDM stream instead
> [4][5].
> That's the hd tv data stream the chip RTL2832U [5] in the rtl-sdr is meant
> to demodulate in hardware.

I hadn't actually considered using the built-in de-modulation :-S I'll look
into that.

> .. I even found an opencore for ofdm modulation [7], which could (in
> theory) be loaded on an fpga ..

Interesting. Thanks for all the links!

> Cheers,
> Matthieu
> [1] http://en.wikipedia.org/wiki/Project_25
> [2] http://www.maximintegrated.com/app-notes/index.mvp/id/686
> [3] http://nuand.com/
> [4]
> https://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing
> [5] https://www.youtube.com/watch?v=TQ0KDjxhXNk
> [6]
> http://www.realtek.com.tw/products/productsView.aspx?Langid=1&PNid=22&PFid=35&Level=4&Conn=3&ProdID=257
> [7] http://opencores.org/project,ofdm
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